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Gardeners have been urged to wash their hands immediately after using compost

NHS Lothian has said it is investigating four cases of Legionella longbeachae linked to gardening compost.

Two patients are currently being treated in intensive care in hospital. Another two have been discharged after treatment.

Symptoms of the infection can include headaches, diarrhoea or a dry cough followed by pneumonia.

The health board is trying to identify the source of the outbreak.

All four patients are said to be keen gardeners and aged between 64 and 82. They had all recently bought different products containing gardening compost before they became infected.

Public health experts believe the outbreak is linked to commercial gardening compost.

The exact way in which the infection is passed from compost to people is not known, but it assumed to be through breathing in very small dust particles or very small drops of contaminated water.

I would like to add further reassurance that the risk to the wider public is lowDr Richard Othieno, NHS Lothian

Most people recover after treatment with antibiotics but those with underlying medical problems are more vulnerable.

Dr Richard Othieno, consultant in public health and chair of the incident management team, said: "This type of Legionella is quite rare in that unlike other strains it has never been identified in man-made water systems, like cooling towers.

"We are working with experts to trace the source of the infection and samples of the compost have been sent for testing.

"We know that each of the four cases are keen gardeners and had purchased different products containing compost prior to acquiring the infection.

"Gardening is a healthy hobby but there are risks and it is important that people take some simple precautions when working in their garden or with gardening products.

"I would like to add further reassurance that the risk to the wider public is low."

The health board has published advice to minimise the risk to anyone handling garden mixes (bagged or unbagged) such as potting mix, mulches, composts and garden soils:

• read and follow any manufacturers' instructions on the bag

• open any compost or potting mix bags carefully, if possible using a blade

• wear gloves when handling compost

• keep the door open in greenhouses or sheds when potting-up plants or filling hanging baskets

• wear a mask if the air is dusty, particularly indoors

• wash hands immediately after use of compost

• if you are going to smoke while gardening, please wash your hands before doing so.